1. Field of the Invention
The present invention relates to the field of electric power switches, and more particularly, to the field of light, fast acting switches.
2. Prior Art
Electrical switches come in many different configurations. In general, switches include a fixed contact and a movable contact. When the fixed and movable contacts are in contact, the circuit controlled by the switch is closed. To open the circuit, the movable contact is actuated or withdrawn away from the fixed contact. When current is flowing in the circuit at the time the switch is opened, an arc develops across the contacts as the switch opens. This arc is extinguished by a variety of different mechanisms, depending on switch design. In some switches designed for AC use, this arc extinguishes in the vicinity of the next zero crossing of the current waveform as a consequence of the arc current reducing to a value at which the arc becomes unstable. It is an inherent property of arcs that when the arc current becomes small enough, an instability develops and the arc extinguishes. In some devices such as those used to interrupt DC currents which have no zero crossings, the arc is intentionally lengthened to render it unstable. Such lengthening causes the arc voltage to increase about 30 volts for each centimeter increase in arc length where the contacts are disposed in an air atmosphere at normal atmospheric pressure. This is added to the voltage drop of the electrodes which is always present and may range from about 10 volts to about 30 volts depending upon what materials are present in the electrodes. The source voltage which serves to drive current through the circuit to be interrupted is in effect reduced by the magnitude of the arc drop, thereby reducing the circuit current. If the arc drop can be increased to approach or equal the source voltage, the current becomes very small or zero and the arc becomes unstable and extinguishes, thereby interrupting the circuit. Frequently, a magnetic field is used to stretch out the arc sufficiently to render it unstable so that it extinguishes.
In some relays, in order to avoid the problems associated with carrying a main current conductor on the moving arm of the relay, the relay is provided with a bridging contact which bridges a pair of fixed contacts which are connected to the main load terminals. With the bridging member disposed in physical contact with the spaced apart main contacts, the circuit is closed. Upon actuation of the relay to open the circuit, the shorting bar is retracted. As the shorting bar retracts, two gaps open in the load circuit, one between the first fixed contact and the shorting bar and a second one between the shorting bar and the second fixed contact. In such relays, arc extinction is accomplished in a manner similar to single contact-gap switches.
Piezoelectric bender relays have now been developed for use in communication and other circuitry. See, for example, U.S. Pat. No. 4,697,118 entitled "Piezoelectric Switch" by Harnden, Jr., et al. issued Sept. 29, 1987 and U.S. patent application Ser. No. 244,647 filed Sept. 13, 1988 as a continuation of U.S. patent application Ser. No. 103,753 filed Dec. 9, 1987 as a continuation-in-part of application Ser. No. 911,171 filed Sept. 24, 1986 by W. P. Kornrumpf, both incorporated herein by reference.
Most simple switches designed to operate with current loads up to 10 amps have gap lengths, when fully open, of a few millimeters or less. This includes both electromagnetically and piezoelectrically driven devices. Such switches are not considered useful in controlling DC power circuits because the voltage developed by the arc across the short gap spacing between fully open contacts is limited to contact voltage which usually is insufficient to limit current from say a 110 volt source to a level which can be interrupted.
Such devices are used successfully in low power AC circuits because the current periodically passes through sinusoidal zeros which allow interruption to occur. But even in this case, the arc drawn as the contacts part will in general persist until the end of the power half cycle causing contact erosion. Such erosion can be substantially reduced or even essentially eliminated if the current is depressed nearly to zero at the moment the contacts open. A switch capable of such current suppression regardless of the instant in a power half cycle at which the contacts open is the subject of the present invention.